1. Field of the Invention
The invention is related to a distance monitoring device and more particularly to a distance monitoring device used in a chemical mechanical polishing apparatus.
2. Description of Related Art
Because the resolution of photolithography exposure increases along with the decrease in device size and the depth of field at exposure is reduced, the requirement to evenness of chip surface increases drastically. Thus, when performing the deep sub-micron process, the planarization of the chip then depends on the chemical mechanical polishing (CMP) process. The unique anisotropic polishing property of the CMP process is not only used for the planarization of the surface contour of the chip, but can also be applied in the fabrication of damascene structures of perpendicular and horizontal metal interconnections, the fabrication of shallow trench isolations in devices and the fabrication of advanced devices in the previous stage, and the fabrication of micro-electromechanical system planarization and the fabrication of flat displays, etc.
The CMP process mainly utilizes a reagent in the polishing slurry for generating a chemical reaction on the front side of the wafer to form a polishable layer. Further, with the wafer on the polishing pad, the protruding portions of the polishable layer are polished off by the mechanical polishing with the facilitation of abrasive particles in the polishing slurry. The chemical reactions and the mechanical polishing are then repeated to form a planar surface.
Nevertheless, the bending level of the wafer becomes higher as the rigidity of the wafer becomes lower. When the wafer bends, the polishing rates in various regions of the wafer are different under the CMP process, such that the surface of the wafer has unfavorable flatness.